Support textTransform capitalize enum

_I am..._

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| Requesting a new feature
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**Proposal/Feature-request:**

Support `capitalize` enum in the CSS `textTransform` property for React Native and web feature parity, as mentioned in #495 .

I think this will need to be a manual transformation applied to the text string children, and we need to decide what `TextTransform` enum will be passed onto the Sketch file: `none` is probably best.

No source file or test is named. Start by finding the textTransform handling for text string children, review the related request in #495, and determine which TextTransform value is sent to the Sketch file; done means capitalize renders correctly for React Native and web feature parity with the chosen Sketch behavior documented and tested.

Written by the indexing model from the issue text.